This invention relates to very compact integrated circuits and particularly to integrated filters.
A frequency selecting filter serves three important functions, namely, passing electrical signals at a desired frequency, rejecting other frequencies, and matching both source and load impedances. The ongoing trend of miniaturization through integration presents a problem in filter matching circuit design, in which parasitic shunt capacitance prevents the filter from being matched to its source and load, and also presents a problem in filter stopband isolation design, in which the coupling between the filter""s input and output (I/O) sections compromises the filter""s stopband rejection performance.
In conventional filter design, the parasitic shunt capacitance is small, and its effect on the matching circuit performance is mostly negligible. Also, in conventional filter design, the coupling between a filters I/O sections can be reduced by physically separating these two sections so that the impact on the filter""s stopband rejection performance is negligibly small. However, as circuit design moves to highly integrated circuits, such multilayer ceramic integrated circuits (MCIC) or multilayer printed circuit boards (MPCB) and high density interconnects (HDI), the parasitic shunt capacitance adversely affects the matching circuit performance. This is relatively new and challenging problem in modern integrated filter design, and the prior art approach, which is to increase spacing between filter elements and to use ground planes, is not a viable solution in integrated filter design.